It is absolutely easy to switch from a specialization to core. Just go to the Dean's office and tell them you wanna switch and your done. The other way around, not so easy, next to impossible they say, lol. Unless you're getting like 12 in everything (which is not likely), I doubt you can switch from core to specialization. Unfortunately, second year biochem is the same for both core and specializations, so you don't really get a clear idea of what your specialization is like. BUT, the plus is, you'll definitely know by the end of the year if you wanna do biochem from a research perspective, in which case you should stay in specialization, or if you wanna do biochem, but avoid labs, in which case you should switch to core which doesn't have anymore lab courses. So I suggest you choose the specialization and then make your decision about switching at the end of specialization. I'm in the biomed spec. and I'm going into third year. I found that although lab work was tough, it was really helpful in understanding the stuff you learn in class, so I decided to stay in the spec. Plus, in 3rd and 4th year, there are some courses which you can only take IF you're in specialization. Anotehr bonus is that you HAVE TO be in a specialization to get into the biochem co-op program. So if the co-op option is in the back of your head..stay in specialization.
Hope that helps